It is well known in the art of electrographic printing and electrophotographic copying to form an electrostatic latent image corresponding to either the original to be copied, or corresponding to digitized data describing an electronically available image.
In electrophotography an electrostatic latent image is formed by the steps of uniformly charging a photoconductive member and imagewise discharging it by an imagewise modulated photo-exposure.
In electrography an electrostatic latent image is formed by imagewise depositing electrically charged particles, e.g. from electron beam or ionized gas onto a dielectric substrate.
The obtained latent images are developed, i.e. converted into visible images by selectively depositing thereon light absorbing particles, called toner particles, which usually are triboelectrically charged.
In toner development of latent electrostatic images two techniques have been applied: "dry" powder and "liquid" dispersion development of which dry powder development is nowadays most frequently used.
in dry development the application of dry toner powder to the substrate carrying the latent electrostatic image may be carried out by different methods known as, "cascade", "magnetic brush", "powder cloud", "impression" or "transfer" development also known as "touchdown" development described e.g. by Thomas L. Thourson in IEEE Transactions on Electronic Devices, Vol. ED-19, No. 4, April 1972, pp.495-511.
In most cases the latent image is developed with a finely divided developing material or toner to form a powder image which is then transferred onto a support sheet such as paper.
The transferred image may subsequently be permanently affixed to the substrate by heat, pressure, or a combination of heat and pressure.
Electrophotographic processes can not only be employed to form monochrome (black) images, but also to form colour images. It is known to form full colour images by sequentially forming and developing electrostatic colour sparation images with cyan, magenta, yellow and black toners respectively. In such applications high quality toners are needed.
By `quality` in electrostatography is generally understood a true, faithful reproduction of the original to be copied, or faithful visual print of the electronically (digitally) available image.
Quality comprises features such as uniform darkness of the image areas, background quality, clear delineation of lines, good resolution of the image and particularly for colour images correct hue, high saturation and high lightness.
Recently the need for reproducing, with offset quality, not only line originals but also halftone originals or a combination of both by electrostatographic processes has steadily been rising. This means that the electrostatographic process must be able to reproduce faitfully both fine lines (i.e. have high resolution) and uniform density areas with low as well as high density and this with fairly low differences in density (i.e. having a good and stable gray scale balance).
It is known that to achieve high resolution imagiges by an electrostatographic system using one of the important contributing characteristics of high quality electrostatographic developers is the size and size distribution of the toner particles used as developing particles in case of a single component developer, and in case a two component developer material is used, in particular the size and size distribution of the toner particles employed. In the document published by ATR Corporation, 6256 Pleasant Valley Road, El Dorado, Calif. 95623, entitled `Effect of Toner Shape on Image Quality` published Mar. 28, 1988, the influence of toner particle diameter and shape upon image quality, particularly for high resolution images, has been tested. Examples of toners comprising small particles with a narrow size distribution are disclosed in e.g. U.S. Pat. Nos. 4,748,474; 4,737,433; 4,434,220; 4,822,60 and WO A1 91/00548.
To improve further the quality of the developer, toner particles with volume average grain size lower than 4 to 5 .mu.m and showing a narrow size distribution should be used. Although there are many processes to produce toner particles (e.g. by melt kneading all ingredients), few produce such toner particles having a narrow size distribution. If the production process itself does not yield a narrow size distribution, the toner particles have to be sized through classification. The efficiency of this classification process is strongly determined by particle size. The smaller the particle size the less efficient the classification process. Toner particles with an average size of less than 5 .mu.m and narrow distribution are difficult to obtain. Such fine toner particles present a high production cost.
It is known that to produce a developer capable of yielding high electostatographic quality it is necessary to match the grain size of the carrier particles to the grain size of the toner particles. Examples of this reasoning can be found in e.g. U.S. Pat. No. 3,942,979 and EP 004748. Both these documents disclose that once the particle size of carrier and toner particles are matched any carrier, coated or uncoated can be used.
The importance of adapting the properties of toner particles and carrier particles such that both are matched, has been disclosed in DE-OS 3,549,358. A possible way to adapt the properties of the carrier particles to the properties of the toner partilces is to coat the former with a resin so as to maximize the overall developer performance of the carrier/toner combination.
The use of a polytetrafluorethylene (PTFE) coating on carrier particles that are used in combination with toner particles with small particle size is known to be beneficial. In U.S. Pat. No. 4,434,220 however it is disclosed that the PTFE coating is too sensitive to abrasion, giving toner contamination by fluorocarbons and hence an appreciable shift in properties of the toner particles. In U.S. Pat. No. 4,434,220 it is disclosed that this problem can be prevented by coating the carrier particles with a complex ternairy coating of polytetrafluoroethylene, fluorinated ethylene-propylene and poly(amide-imide).
Another way to further improve the quality of an electrostatographic developer is to lower the particles size of the carrier particles used in a two component developer as disclosed e.g. in EP 004748. However, the mere reduction of the size of all carrier particles, without special precautions, introduces problems. The magnetic attraction of smaller carrier particles is largely reduced, which gives an appreciable increase in the risk of carrier loss. By merely reducing the size of all carrier particles, the number of carrier particles, present in the developer composition is increased. This means that also between the magnetic roller surface and the latent image bearing member more carrier particles, surrounded by insulating toner particles, are present; this increases the electrical resistivity of the magnetic brush, reduces the field effect and enhances the edge effect, which is totally unwanted in high quality images. It is possible to overcome said problem of edge effects, by increasing the conductivity of the carrier particles, but it is not possible to vary the conductivity of the carrier particles within broad limits, since an increase in conductivity of the carrier particles gives also an increase of the risk of charge injection phenomena in the carrier particle, due to the electric field of the development, which again increases the risk of carrier loss.
The use of fine toner particles in itself and especially in combination with fine carrier particles, poses additional problems. The smaller the toner particles, the higher the electrostatic charge aquired by the toner particles during the triboelectric contact between toner and carrier particles. Since electrostatographic development can be looked upon as a (partial) charge neutralization of the electrostatic latent image on the latent image bearing member by oppositely charged toner particles, the electrostatic charge of the latent image is neutralized by a small number of toner particles when using highly charged toner particles. This results in low maximal optical densities in the image. To overcome this problem, it is necessary to use a higher development field (i.e. keeping the latent image bearing member on a higher electric potential), which again increases the risk of carrier loss. A higher development potential poses also problems of faster deterioration of the latent image bearing member, e.g. photoconductors.
Carrier loss must be avoided when using an electrostatographic system to reproduce faitfully both fine lines (i.e. have high resolution) and uniform density areas with fairly low differences in density (i.e. having a good gray scale balance). When carrier particles are deposited together with toner particles on the latent image to form a powder image that will be transferred on the support for the final image, they increase the distance between the latent image bearing member and the final support and hampers the adequate transfer of the powder image to the final support. Moreover around the carrier particles no transfer at all takes places leaving white spots in the final image. On the other hand, carrier particles being mostly black, in those places where carrier particles are accidentally transferred along with toner particles, black spots are present in the final image. These blemishes are intorerable when reproducing high quality, half tone, full colour images.
Although all disclosures concerning the matching of carrier and toner particles to achieve high electrostatographic quality do provide improvements for developers, there is still an appreciable need for further improvement in the production of two component developers for electrostatographic application where "offset-quality" is desired in the final copy. By "offset-quality" is meant a print quality that is indistinguishable from the quality that can be attained by classical offset printing techniques. Especially the need to have a developer, with which it is possible to combine high resolution, highly uniform optical density, full gray scale control and low defects such as low carrier loss, is still present. A "fine-haired" magnetic brush with low carrier loss with an extended life cycle for both photosensitive member and developer, is still not totally attainable with the cited teachings.